Pacific gray whales have decreased by 13% so far this century

June 14 () –

Gray whales that spend their summers feeding in the shallow waters of the Pacific Northwest Coast of the USA have suffered a 13 percent decline in length this century.

According to a new study from Oregon State University (OSU), the smallest size could have important consequences for the health and reproductive success of affected whalesand also raises alarm bells about the state of the food web in which they coexist, the researchers say.

“This could be an early warning sign that the abundance of this population is starting to decline, or is unhealthy,” he said. it’s a statement KC Bierlich, co-author of the study and associate professor at the OSU Marine Mammal Institute. “And whales are considered sentinels of the ecosystem, so if the whale population is not doing well, That could say a lot about the environment itself.”

The study, published in Global Change Biology, analyzed the Pacific Coast Feeding Group (PCFG), a small subset of about 200 gray whales within the larger Eastern North Pacific (ENP) population of about 14,500. This subgroup stays closer to the Oregon coast and feeds in warmer, shallower waters than the Arctic seas, where the bulk of the gray whale population spends most of the year.

Recent OSU studies have shown that whales in this subgroup are smaller and have overall poorer body condition than their ENP counterparts. The current study reveals that they have been shrinking in recent decades.

The Marine Mammal Institute’s Geospatial Ecology of Marine Megafauna (GEMM) Laboratory has been studying this subgroup of gray whales since 2016, including flying drones over the whales to measure their size. Using images of 130 individual whales with known or estimated ages between 2016 and 2022, the researchers determined that an adult gray whale born in 2020 is expected to reach an adult body length 1.65 meters shorter than a gray whale born before 2000. For PCFG gray whales that grow to be between 13 and 14 meters long at full maturity, that represents a loss of more than 13% of its total length.

If the same trend occurred in humans, that would be like the average American woman’s height dropping from 1.53 to 1.40 over the course of 20 years.

“In general, size is critical for animals,” said Enrico Pirotta, lead author of the study and a researcher at the University of St. Andrews in Scotland. “It affects their behavior, their physiology, their life history and it has cascading effects for the animals and for the community they are a part of.

Whales that are smaller at weaning age may be unable to cope with the uncertainty that comes with being newly independent, which can affect survival rates, Pirotta said.

For adult gray whales, one of the biggest concerns is reproductive success. “Being smaller, there are questions about how effectively these PCFG gray whales can store and allocate energy to grow and maintain their health. It is important to know if they can allocate enough energy to reproduction and maintain population growth,” Bierlich said.

Scars on PCFG whales from boat strikes and entanglements in fishing gear also worry the team that a smaller body size with lower energy reserves may make whales less resistant to injury.

The study also examined patterns of the ocean environment that likely regulate food availability for these gray whales off the Pacific coast by tracking “upwelling” and “relaxation” cycles in the ocean. Upwelling carries nutrients from deeper to shallower regions, while periods of relaxation allow those nutrients to remain in shallower areas where light allows plankton and other tiny organisms to grow. including the prey of gray whales.

“Without a balance between upwelling and relaxation, the ecosystem may not be able to produce enough prey to support the large size of these gray whales,” said co-author Leigh Torres, associate professor and director of the GEMM Laboratory at OSU.

The data shows that the size of the whales decreased simultaneously with changes in the balance between upwelling and relaxation, Pirotta said.

“We haven’t looked specifically at how climate change is affecting these patterns, but in general we know that climate change is affecting the oceanography of the Northeast Pacific through changes in wind patterns and water temperatures,” he said. “And these factors and others affect the dynamics of upwelling and relaxation in the area.”

Now that they know that gray whales’ body size is decreasing, researchers say they have many new questions about the consequences of that decline in current and the factors that might be contributing to it.